Measurement system

Abstract

A fluid measurement system includes two transducers which are located in a duct to provide a measuring path through a measuring tube. The transducers are switchable to allow each to be used for both transmission and reception and the speed of fluid passing through the measuring tube will be depedent on the difference in time periods for a signal from the transducers passing with or against the fluid flow. A calibration mechanism employing an auxiliary transducer with its own ultrasonic paths allows accurate measurment of speed of sound values to be determined to the particular system parameters including transducer delay.

I claim: 
     
       1. Apparatus for measuring the flow of a fluid, comprising; a duct with an inlet and outlet for the fluid;   a measuring tube within the duct;   first and second transducers disposed in the duct to define an acoustic path through the measuring tube;   switching means for allowing each transducer to be used for either transmission or reception of a signal used to measure the speed of the fluid through the measuring tube; and   correction means for substantially eliminating transducer delay errors to ensure the accuracy of the fluid speed measurement.   
     
     
       2. Apparatus as claimed in claim 1, wherein the correction means is configured to measure the time for a transmitted signal to pass along two paths of known length. 
     
     
       3. Apparatus as claimed in claim 2, wherein each path includes at least one reflective surface to cause said signal to return at least one to a point from which the signal originated. 
     
     
       4. Apparatus as claimed in claim 2, wherein the correction means includes an ultrasonic transducer for transmitting the signal down the two paths. 
     
     
       5. Apparatus as claimed in claim 4, wherein the transducer is provided in addition to the first and second transducers. 
     
     
       6. Apparatus as claimed in claim 4, wherein the transducer is provided with acoustic paths separate to that within the measuring tube. 
     
     
       7. Apparatus as claimed in claim 6, wherein the acoustic paths are provided within a chamber having a restricted inlet and outlet to allow only a small portion of fluid to be utilised for calibration correction relative to the measurement tube. 
     
     
       8. Apparatus as claimed in claim 7, wherein the chamber includes a reflective surface at the end remote from the transducer. 
     
     
       9. Apparatus as claimed in claim 6, wherein the acoustic paths are provided within a chamber having a plurality of reflective surfaces to cause the acoustic paths to be provided within a relatively small region. 
     
     
       10. Apparatus as claimed in claim 7, 8 or 9, including computation means for providing a calibration correction value dependent on the difference in times for a signal to pass from the transducer to the reflective surface on more than one journey. 
     
     
       11. Apparatus as claimed in claim 10, wherein the computation means is configured to determine time windows for reception of a signal to avoid detection of unwanted reflected signals. 
     
     
       12. Apparatus as claimed in claim 10, wherein the computation means is configured to carry out the correction at less frequent intervals to that of the speed measurement. 
     
     
       13. Apparatus as claimed in claims 7 or 8, wherein the chamber is provided within the duct in a position parallel to the measuring tube. 
     
     
       14. Apparatus as claimed in claim 13, wherein the chamber and the measuring tube each have first and second portions contained within separated portions of the duct associated with the inlet and outlet of the fluid respectively. 
     
     
       15. A method for measuring the flow of fluid through a duct, said method comprising: transmitting a signal from a first transducer through the flow path of the fluid for receipt by a second transducer;   switching the transducers to allow transmission to be effected by the second transducer for receipt by the first transducer;   determining the speed of flow of the fluid from the transducer signal transmission times; and   performing a correction measurement for removing transducer delay errors to ensure the accuracy of the fluid speed measurement,   wherein the correction step is performed by measuring the time for a signal transmitted by a third transducer to pass down two paths of known lengths separate to those from the first and second transducers.   
     
     
       16. A method as claimed in claim 15, including reflecting the signal from at least one reflective surface to return the signal to a point from which the signal originated. 
     
     
       17. A method as claimed in claim 16, including computing a calibration correction value dependent on the difference in times for a signal to pass from the third transducer to at least one reflective surface on more than one journey. 
     
     
       18. A method as claimed in claim 17, wherein the computing step includes providing a time window for reception of a signal to avoid detection of unwanted reflected signals. 
     
     
       19. Apparatus for measuring the flow of a fluid, comprising; a duct with an inlet and outlet for the fluid;   a measuring tube within the duct;   first and second transducer means disposed in the duct to define an acoustic path through the measuring tube;   switching means for allowing each transducer means to be used for either transmission or reception of a signal used to measure the speed of the fluid through the measuring tube; and   third transducer means for providing a periodic correction measurement by determining the time for a transmitted signal therefrom to pass down two paths of known length for calibration purposes to ensure the accuracy of the fluid speed measurement performed by the first and second transducer means.   
     
     
       20. Apparatus as claimed in claim 19, wherein the third transducer means includes at least one reflective surface associated therewith to provide two paths of dissimilar overall length. 
     
     
       21. A method for measuring the flow through a duct, said method comprising; transmitting a signal from a first transducer through the flow path of the fluid for receipt by a second transducer;   switching the transducers to allow transmission to be effected by the second transducer for receipt by the first transducer;   determining the speed of flow of the fluid from the transducer signal transmission times; and   periodically performing a correction measurement by utilising a third transducer for determining the time for a transmitted signal therefrom to pass down two paths of known length for calibration purposes to ensure accuracy of the fluid speed measurement effected by the first and second transducers.   
     
     
       22. A method as claimed in claim 21, wherein the correction step is performed by utilising at least one reflective surface to provide two paths of dissimilar overall length.